If an exchange line call has been set up in a telephone, a so-called inquiry call can be initiated by operating the grounding key. The exchange line interface maintains the existing call and switches the inquiries of the extension to an inquiry call subscriber line circuit. From the inquiry call subscriber line circuit, an internal call can be set up. The exchange line call can be transferred to another subscriber via this inquiry call path.
In digital switching systems, the telephone set is connected to an integrated semiconductor chip which handles the so-called BORSCHT functions. This integrated semiconductor circuit is generally called an SLIC (subscriber line interface circuit) circuit. “BORSCHT” is an artificial word to describe more easily the functions of a subscriber line circuit within a switching center. The word “BORSCHT” is composed of the initial letters of the designation for the various functions of such an SLIC semiconductor circuit. These functions include the battery feed, the overvoltage protection, the ringing, the signaling, the coding, the hybrid and the provision for testing.
In U.S. Pat. No. 5,659,570, an integrated SLIC circuit is described which contains a grounding key detection circuit. In this arrangement, the SLIC circuit is connected to the telephone set with grounding key via two telephone connecting lines, the grounding key detection circuit contained in the SLIC circuit detecting an unbalance of the currents flowing in the two connecting lines to output a grounding key detection signal when the grounding key is operated at the telephone set. The two currents flowing in the connections are in each case compared with reference currents which are generated by two reference current sources. The two reference currents generated by the reference current sources are of equal magnitude. If the current IT (T: tip) in the first telephone set connecting line is greater than the sum of the current IR (R: ring) flowing in the second telephone set connecting line and the reference current generated by the first reference current source or if the current IR flowing in the second telephone connecting line is greater than the sum of the current IT flowing in the first telephone connecting line and the reference current generated by the second reference current source, the grounding key detection circuit of the integrated SLIC circuit detects an operation of the grounding key of the telephone set and outputs a grounding key detection signal.
Though current interference signals on the telephone connecting lines do not lead to false grounding key detections, the imbalance of the currents IT and IR flowing in the telephone connecting lines must persist for a certain period of time. An imbalance of the two currents flowing in the telephone connecting lines is also called a longitudinal current. As soon as this longitudinal current exceeds a particular threshold current for a particular guard period, a grounding key detection signal is output in such conventional grounding key detection circuits.
In grounding key detection circuits according to the prior art, a disadvantage exists, nevertheless, in that sinusoidal current disturbances in the telephone connecting lines which are capacitively coupled into the lines could lead to the current threshold being undershot again for a short period of time before the period of guard time has elapsed. If, for example, the guard period is 4 ms and the detected longitudinal current drops below the current threshold due to a superimposed sinusoidal interference current before the guard period of 4 ms is reached, the operating of the grounding key is not detected even if the longitudinal current exceeds the current threshold again within a very short time after it has dropped below it. The capacitively injected sinusoidal interference currents can periodically push the longitudinal current produced below the threshold value at the wave dips due to the operating of the grounding key before the set guard time has in each case elapsed. The operating of the grounding key is not detected in such a case.